Radio frequency identification (RFID) technology is becoming increasingly important for logistics concerns, material handling and inventory management in retail stores, warehouses, distribution centers, buildings, and like controlled areas. An RFID system typically includes at least one RFID reader, also known as an RFID interrogator, and an RFID tag that is usually attached to, or associated with, an individual item, or to a package for the item. The RFID reader interrogates one or more RFID tags in its coverage range by transmitting a radio frequency (RF) interrogating signal, and the RFID tag, which senses the interrogating RF signal, responds by transmitting a return RF signal. The RFID tag either generates the return RF signal originally, or reflects back a portion of the interrogating RF signal in a process known as backscatter. The return RF signal may further encode data stored internally in the tag. The return signal is demodulated and decoded into data by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), or any combination of attributes, and so on.
The RFID tag typically includes an antenna, a power management section, a radio section, and frequently a logic section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. An RFID tag with an active transmitter is known as an active tag. An RFID tag with a passive transmitter is known as a passive tag and backscatters. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. An RFID tag that backscatters and is powered by an on-board battery is known as a semi-passive tag.
The RFID system is often used in an inventory monitoring application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to position at least one RFID reader in a controlled area, and then, to allow each reader to automatically read whatever tagged items are in the coverage range of each reader. For superior RF coverage, it is known to provide each reader with a phased antenna array that generates an interrogating beam that is electronically steered both in azimuth, e.g., over an angle of 360 degrees, and in elevation, e.g., over an angle of about 90 degrees.
As advantageous as such known inventory-taking RFID systems utilizing phased antenna arrays have been, it has proven difficult in practice to very accurately and rapidly locate a particular tag. There is a practical limit on the number of antennas that can be used in the array. This antenna limit causes the interrogating beam to have a relatively broad beam width. The interrogating beam is typically steered to a steering angle at which the reader reads the tag with the highest or peak receive signal strength (RSS). However, determining the location, i.e., the azimuth and the elevation, of a tag based on the peak RSS of the interrogating beam is imprecise due to the relatively broad beam width of the interrogating beam.
Rather than relying on the peak RSS, it has been suggested in other, non-RFID, phased array applications to steer a beam by null steering techniques. Nulls are generally “sharper”, i.e., vary more over a given steering angle, as compared to peak RSS steering for most antenna radiation patterns. Hence, a null does not suffer from the drawback of a broad beam width. However, null steering techniques are not very useful for RFID readers, since the tag must be irradiated with a certain minimum strength interrogating RF signal in order to power the tag and enable it to be read. Additionally, a certain minimum strength return RF signal is needed to demodulate the tag. An RFID tag will not be detectable in a null.
Accordingly, there is a need to more accurately locate RFID tags despite the practical limit on the number of antennas that can be used in a phased antenna array.
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The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.